The present invention relates generally to amplifier output circuits, and in particular to amplifier output circuits fabricated using gallium arsenide (GaAs) field-effect transistors. More particularly, the invention addresses the challenge of implementing a linear, wide band Class AB amplifier with a controllable operating point using single polarity GaAs FET devices.
Known amplifier output circuits include linear class A amplifiers and nonlinear "totem pole" circuits. Class A amplifiers are inefficient and must dissipate large amounts of power, while totem pole circuits have substantially different gains for positive and negative output voltages and are difficult to compensate within a feedback loop. In addition, some forms of totem pole circuits do not provide a means for controlling the bias current in the output devices.
Another class of known output circuits does include a control mechanism to establish the bias current in the output devices. These circuits fall into three categories: One includes designs that provide an adequate control of the bias current in the output devices, but which require the use of both n-channel and p-channel devices. Circuits of the second category are designed with single polarity devices, but the control of the output bias current is poor because, typically, a control voltage affects only one of the output devices. The third category of such circuits provides adequate bias current control, but at the expense of severely limited output drive capability and wasted power.
Accordingly, a general objective of the present invention is to overcome these and other drawbacks of known amplifier output circuits by providing a class AB output circuit fabricated with single polarity GaAs devices, wherein the bias current in both output devices is controlled, the positive and negative gains are similar, and cross-over distortion is minimized while maintaining adequate output drive capability.